[Illustration: FIG. 2.]
7. In Fig. 2 we have represented two equal weights to which strings
are attached; these strings, after passing over pulleys, are fastened
by a knot C. The knot is pulled by equal and opposite forces. I mark
off parts CD, CE, to indicate the forces; and since there is no reason
why C should move to one side more than the other, it remains at rest.
Hence, we learn that two equal and directly opposed forces counteract
each other, and each may be regarded as destroying the motion which
the other is striving to produce. If I make the weights unequal by
adding to one of them, the knot is no longer at rest; it instantly
begins to move in the direction of the larger force.
8. When two equal and opposite forces act at a point, they are said to
be in _equilibrium_. More generally this word is used with reference to
any set of forces which counteract each other. When a force acts upon
a body, at least one more force must be present in order that the body
should remain at rest. If two forces acting on a point be not opposite,
they will not be in equilibrium; this is easily shown by pulling the
knot C in Fig. 2 downwards. When released, it flies back again. This
proves that if two forces be in equilibrium their directions must be
opposite, for otherwise they will produce motion. We have already seen
that the two forces must be equal.
A book lying on the table is at rest. This book is acted upon by two
forces which, being equal and opposite, destroy each other. One of
these forces is the gravitation of the earth, which tends to draw the
book downwards, and which would, in fact, make the book fall if it were
not sustained by an opposite force. The pressure of the book on the
table is often called the _action_, while the resistance offered by the
table is the force of _reaction_. We here see an illustration of an
important principle in nature, which says that _action and reaction are
equal and opposite_.